Present day personnel inspection systems designed to locate weapons which are employed at the portals of secured areas, e.g. courthouses, military installations and the like, normally rely upon electromagnetic detection of a mass of metallic material. Such systems have been in use in airports for a number of years. However, the limitations of such systems are becoming increasingly significant. Electromagnetic systems are limited to the detection of metallic items such as conventional handguns and therefore cannot detect the plastic and ceramic weapons now being manufactured and sold. Such electromagnetic systems also cannot form an image of the detected material; they merely respond to a mass of the metal passing the detector. Similarly, such systems are incapable of detecting other contraband, such as drugs or certain chemical explosives.
The prior art includes a number of proposed systems for detection of non-metallic weapons and other contraband. Many of these have relied upon the ability of millimeter waves (radiation of wavelength between one millimeter and one centimeter, that is, between 30-300 GHz frequency) to penetrate clothing without harm to the wearer. Millimeter waves are generally reflected from metallic objects and can be used to form an image of such objects. The attenuation and reflection characteristics of ceramic and plastic weapons, as well as contraband such as narcotics, are different with respect to millimeter-wave radiation from those of skin, so that it is possible, although it has not previously been practical, to form an image of objects of these materials carried by a person. These characteristics render millimeter waves suitable for detection of ceramic weapons or other contraband concealed beneath the clothing, for example, of an individual seeking to enter a secured area.
However, proposed prior art millimeter have contraband detection techniques have not been implemented in practical systems. Prior art systems, as exemplified in a number of documents discussed hereinafter, have in general involved a single millimeter wave detector, which is mechanically scanned over a field of view within which the individual is constrained to stand. The reflected signal can be processed and used to generate a television display. The systems of the prior art have at best required about 30 seconds to generate a suitable image. Clearly this is unsatisfactory for use in airports and other crowded areas where many individuals must be permitted to enter the secured area rapidly. Further, any image which requires a 30 second exposure is highly likely to suffer from blurring as the subject moves. Therefore, such systems have found little use.
Another defect of the systems proposed in the prior art involves the lack of contrast between non-metallic contraband articles and the skin of the subjects, particularly as compared to the high reflectivity of specular objects, e.g. belt buckles, eyeglasses, coins, watches and the like.
References generally showing proposed contraband detection systems as described above include the following:
T. S. Hartwick, D. T. Hodges, D. H. Barker, and F. B. Foote, "Far Infrared Imagery", Applied Optics 15, 1919, (1976) discusses far infrared (FIR) (i.e. 50-1000 um, or 0.05-1 mm) imaging using an FIR laser source and a helium cooled GaAs detector. Both transmissive and reflective arrangements are described. An image is provided by physically moving the object to be imaged to be scanned with respect to the detection system. While it is suggested that such a system could be used for inspection of persons, it would obviously be impractical for airport scanning and comparable security applications, as it would take on the order of minutes to scan each passenger.
D. T. Hodges and E. Reber, "Evaluation of Passive Far Infrared Radiometric Techniques for Detection of Concealed Objects", Aerospace Report ATR-79 (7745)-1 (Mar. 23, 1979) discusses FIR (300-3000 um, or 0.3-3 mm) imaging of radiation for detection of nuclear materials and weapons. A radiometric technique is proposed, in which the temperature of objects in the field of view is measured. The difference between body temperature and ambient temperature is used to locate objects carried by individuals. Detection can be accomplished using either "coherent (heterodyne) or incoherent (video) techniques." The coherent approach was chosen, using a Schottky barrier diode mixer and a klystron local oscillator. Again, a single-element detector was used to scan the subject, so that lengthy periods of time were required to form each image.
It is suggested on page 8 of this paper that while a passive system (in which only the radiation emitted by the subject is measured) is preferable, reasonable rates of imaging might require illumination for improved contrast by incoherent sources such as mercury arc lamps with filters to remove the ultraviolet, visible and infrared emissions. It was also noted that some explosive materials were not readily distinguishable from the skin of the subject.
D. T. Hodges, F. B. Foote, and R. D. Reel, "Feasibility of FIR Detection of Selected Materials", Aerospace Report ATR-77 (7675)-1 (Sept. 30, 1977) simply reports on tests suggesting that common clothing materials are substantially transparent to FIR radiation. A FIR laser and pyroelectric detector were used in these experiments.
E. Reber, F. B. Foote, R. L. Schellenbaum, and R. G. Bradley, "Evaluation of Active and Passive Near Millimeter-Wave Radiometric Imaging Techniques for Detection of Concealed Objects", Sandia National Laboratories Report SAND 81-1051 (July 1981) discusses an FIR or near millimeter wave (NMMW) (300-3000 um, or 0.3-3 mm) imaging system. A two-dimensional mechanical scanning system was used in conjunction with an NMMW radiometer to generate a video signal which drove a TV monitor. Active illumination was provided by a klystron. Incoherent illumination was also proposed, using a blackbody radiating at 77K. The heterodyne detection approach was employed, using a Schottky diode mixer and another klystron as a local oscillator source.
This reference discusses on page 15 that if a point source of illumination is used, large flat objects can fail to be detected if they reflect the illumination energy away from the detector, and states that a practical system will require a suitable illumination source. It is stated that ideally the entire hemisphere in front of the person being imaged would be illuminated. It was concluded that coherent illumination, as supplied by a klystron, was unsatisfactory. See page 19.
The time required for this system to form an image was approximately 2 minutes; this is obviously impractical for an airport personnel inspection system, for example. It was suggested that while this might readily be reduced to 1 minute, further improvements would require development of a detector array not then available. Image processing was also proposed.
The disclosure of E. E. Reber and F. B. Foote, "Evaluation of Active and Passive Near-Millimeter-Wave Radiometric Imaging Techniques for Detection of Concealed Objects", Aerospace Corporation Report ATR-80 (7843)-2, (Mar. 20, 1981) is essentially identical.
R. L. Schellenbaum, "Far Infrared Contraband Detection System Development for Personnel Search Applications", Sandia Report SAND82-0161 (September 1982) follows the work discussed immediately above and investigated better illumination. A microwave Michelson interferometer was used as the experimental apparatus. A hybrid tee divided the radiation from a Gunn diode source between the field of view and a reference arm. The interrogation signal was mechanically scanned over the target. Hence, lengthy periods would again be required to generate an image. The target reflected the signal back to the detector via the hybrid tee. A variable short and attenuator in the reference arm controlled the phase and amplitude of a reference signal received by the Schottky diode detector. This interferometric technique involved some unavoidable sensitivity to source-to-object distance, which was undesirable. See pp. 9-12.
A number of alternative illumination schemes were also suggested (see page 11). Among others, a large number of point sources was considered, but deemed impractical. Incoherent illumination provided by mercury lamps was considered inadequate. A system involving two collimated Gunn diode oscillators and two dispersing elements reflecting the energy onto an inner diffusing surface of a spherical chamber was suggested. This reference also discusses use of wire-grid polarizers to distinguish retroreflected target signal return (i.e. specular reflection from smooth metallic surfaces) from directly reflected body background (diffuse reflection from skin), concluding that the use of polarizers would not be fruitful. See page 39.
J. A. Gagliano, J. M. Cotton, D. M. Guillory, R. H. Platt, and A. T. Howard, "New Weapons Detection Concepts", Final Report on Project A-4666, Georgia Institute of Technology (February 1988) discloses detection of non-metallic weapons. Acoustic, infrared and millimeter wave systems were tested; only the last was found to be feasible. A millimeter wave (3 mm) imaging system comprising a single mechanical scanner was used for evaluation of passive ;nd active systems. This again would involve lengthy image generation delays. The passive system could not be made to work with the instrumentation available. The active system (i.e. one using illumination sources) comprised an IMPATT oscillator the output of which was frequency modulated and divided in a hybrid tee, which introduced 180.degree. phase difference of the signal between the two transmission ports. It was determined that the active system was unworkable even after the "illumination coherency was further disrupted by sending it from two points with spatially different polarizations" See page 75.
Possible improvements on the active system are discussed at pages 77-78. Development of a single incoherent "noise source" as illuminator is discussed and the suggestion is made that its output energy should be divided and transmitted from several locations, with differing delays, to destroy the effects of "quasi-coherence."
As noted, the Reber et al references suggest that, in principle, array detectors could eliminate scanning and reduce the time required for image formation, but mention that suitable arrays capable of doing so did not exist. Other documents suggest use of multiple element arrays for imaging millimeter wave radiation for astronomical imaging purposes, but contain no suggestion that such arrays might be useful for weapons or contraband detection. See Yngvesson, "Near-Millimeter Imaging with Integrated Planar Receptors; General Requirements and Constraints," in Infrared and Millimeter Waves, 10, (Academic Press 1983). FIG. 8 of this document shows a Vivaldi receptor array. Further details of this detector system are shown in Yngvesson et al, "Millimeter Wave Imaging System with an Endfire Receptor Array", 10th Int'l Conf. on Infrared and Millimeter Waves (1985). Diode detectors extending across pairs of antenna elements making up the detectors of the array are shown in FIG. 7. Similar disclosures are found in Johansson et al, "Millimeter Imaging System with an Endfire Receptor Array", Proc. 15th Europ. Microwave Conf. (1985), in Korzeniowsky et al, " Imaging System at 94GHz using Tapered Slot Antenna Elements," Eighth IEEE Int'l Conf. on Infrared and Millimeter Waves (1983), and in Yngvesson, "Imaging Front-End Systems for Millimeter Waves and Submillimeter Waves," SPIE Conf. on Submillimeter Spectroscopy (1985). The method of injecting the local oscillator signal for heterodyning with the received signal shown in these references, involving injection of the signal into an aperture in a reflector in a Cassegrain-telescope optical system, is very awkward, and the device would not be well suited to contraband detection.
In Stephan et al, "A Quasi-Optical Polarization-Duplexed Balanced Mixer for Millimeter-Wave Applications," IEEE Trans. on Microwave Theory and Techniques, vol. MTT-31, No. 2 (1983) pp. 164-170, a mixer is described in which a local oscillator signal is quasi-optically injected from one side of a substrate and mixed in a balanced mixer with the received signal coupled from the other side. The RF signal and local oscillator (LO) signal can be of orthogonal polarizations. It is suggested that the LO signal could arrive from either side of the substrate, although no implementation of the "same-side" arrangement is provided. The suggestion is made that arrays of such devices could "open the way to phase-coherent imaging of millimeter-wave fields at a focal plane." See page 170. However, neither side of the substrate of this device in the embodiment disclosed would be available for other circuitry, as is highly desirable. There is no teaching in Stephan et al of a practical contraband detection device.